Passenger cars and other vehicles are frequently provided with outward projections extending into the path of the rotating brushes. These include, for example, nonretractable radio antennas, external rear-view mirrors and raised nameplates or other ornamentations. The bristles of a scrubbing brush, especially if sufficiently flexible to hang down from the brush core in the motionless state and to radiate outwardly from that core only during rotation, are liable to become entangled in such structure and to damage same or be damaged by it. The expedient of simply stopping the rotation of the brush as it moves past the obstacle is unsatisfactory since the cessation of the centrifugal force causes the collapse of the bristles and exposes the vehicular surface to impact from the metallic brush core which is being urged toward that surface by its control mechanism. Withdrawing the brush from the vehicular surface in the area of such an obstacle, on the other hand, requires complicated modifications of that control mechanism and, furthermore, prevents the scrubbing of an entire vertical zone adjoining the projection whose avoidance by the brush is intended.